The invention relates to multicarrier transmission systems, and comprises method and apparatus for establishing a power management sleep state in a multicarrier system.
Multicarrier transmission systems provide high speed data links between communication points. Such systems have recently been introduced for communications over the local subscriber loop that connects a telephone service subscriber to a central telephone office; in this important application they are commonly referred to as xe2x80x9cxDSLxe2x80x9d systems, where the xe2x80x9cxxe2x80x9d specifies a particular variant of DSL (digital subscriber loop) communications, e.g., ADSL (asynchronous digital subscriber loop), HDSL (High-Speed Digital Subscriber Loop), etc. These will be referred to generically herein simply as xe2x80x9cDSLxe2x80x9d systems.
In such systems, a pair of transceivers communicate with other by dividing the overall bandwidth of the channel interconnecting the subscriber and the central office into a large number of separate subchannels, each of limited bandwidth, operating in parallel with each other. For example, one common system divides the subscriber line channel into two hundred and fifty six subchannels, each of 4.3 kilohertz bandwidth. A first group of these (e.g., one hundred ninety six) is allocated to communications from the central office to the subscriber (this is known as the xe2x80x9cdownstreamxe2x80x9d direction); a second group (e.g., thirty-two) is allocated to communications from the subscriber to the central office (this is known as the xe2x80x9cupstreamxe2x80x9d direction). The remaining subchannels are allocated to administrative, overhead and control (AOC) functions.
Data to be communicated over the link is divided into groups of bits, one group for each subchannel. The group of bits allocated to a given subchannel is modulated onto a carrier whose frequency is specific to that channel. Typically, quadrature amplitude modulation (QAM) is used for this purpose, and the group of bits is mapped into a vector defined by one of the points of a xe2x80x9cconstellationxe2x80x9d which specifies the allowable data points for transmission over that subchannel at a particular time. Each vector or data point thus comprises a unique symbol representing a specific bit configuration for transmission as a group over its associated subchannel. During the time period allocated for transmission of a symbol (commonly referred to as a xe2x80x9csymbol periodxe2x80x9d or xe2x80x9cframexe2x80x9d), each subchannel transmits its symbol in parallel with all other subchannels so that large amounts of data can be transmitted during each frame.
The number of bits carried by a symbol is dependent on the characteristics of the subchannel over which it is to be transmitted. This may vary from one subchannel to another. The principal determinant is the signal-to-noise ratio of the subchannel. Accordingly, this parameter is measured from time to time in order to ascertain its value for each subchannel, and thus determine the number of bits to be transmitted on the particular subchannel at a given time.
The telephone channel is subject to a number of impairments which must be compensated for in order to ensure reliable transmission. Phase (delay) distortion of the transmitted signal is typically the most limiting of these impairments. This distortion is frequency-dependent, and thus components of a signal at different frequencies are shifted by varying amounts, thereby distorting the signal and increasing the likelihood of erroneous detection unless provision is made to combat it. To this end, frequency domain equalizers (FDQ) and time domain equalizers (TDQ) are commonly incorporated into the transmission channel in order to equalize the phase (time) delay across the channel frequency band. Other impairments also exist. For example, frequency-dependent signal attenuation adversely affects signal transmission on the telephone line. This is compensated by the use of gain equalizers on the line, while echo on the line is handled by the use of echo cancellers.
The problem of signal impairment is especially serious in those xDSL configurations which carry the DSL communications on a common line with ordinary voice communications but which omit the use of a xe2x80x9csplitterxe2x80x9d at either the subscriber premises or the central office or both. A xe2x80x9csplitterxe2x80x9d is basically a filter which separates the low-frequency voice communications (e.g., from zero to four kilohertz) from the higher-frequency data communications (which may extend up into the megahertz band) and provides a strong degree of isolation between the two. In the absence of a splitter, unique provisions must be made to accommodate voice and data communications on the same line. For a more detailed description of the problem and its solution, see the co-pending application of Richard Gross et al. entitled xe2x80x9cSplitterless Multicarrier Modemxe2x80x9d, Serial No. PCT/US98 21442, filed Oct. 9, 1998, and assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference.
Because of their extensive use in Internet communications as well as in other applications, DSL transceivers are commonly maintained in the xe2x80x9conxe2x80x9d state, ready to transmit or receive once they have been installed and initialized. Thus, such modems consume a significant amount of power, even when they are not actively transmitting or receiving data. It is generally desirable to limit this power consumption, both for environmental reasons as well as to prolong the life of the equipment. Further, such modems may be implemented or incorporated in part or in whole in computer equipment such as in personal computers for home and business use, and such computers increasingly incorporate power conservation procedures. See, for example, U.S. Pat. No. 5,428,790, xe2x80x9cComputer Power Management Systemxe2x80x9d, issued Jun. 27, 1995 on the application of L. D. Harper. Thus, it is desirable to provide an ADSL modem which can accommodate power conservation procedures in equipment with which it is associated, as well as independently of such equipment as may be appropriate.
Because of the complexity of DSL transceivers, and the conditions under which they must operate,-it is necessary to initialize them prior to the transmission and reception of data. This initialization includes, inter alia, channel corrections such as xe2x80x9ctrainingxe2x80x9d the frequency-domain and time-domain equalizers and the echo cancellers; setting the channel gains; negotiating the transmission and reception data rates; adjusting the fine gains on the subchannels over which communication is to take place; setting the coding parameters; and the like. Additionally, it includes measuring the signal-to-noise ratio of each of the subchannels, calculating the bit-allocation tables characteristic of each under given conditions of transmission, and exchanging these tables with which other modems with a given modem communicates. For more detailed discussion of these procedures, refer to the application of Richard Gross et al., cited above and incorporated herein by reference. These procedures can require from seconds to tens of seconds. In a new installation, the time required is inconsequential. However, in an already-operating installation, the time required to initialize or re-initialize the system after a suspension of operation in is connection with power conservation is generally unacceptable, since it is typically desired to have the modem respond to request for service nearly instantaneously.
Accordingly, it is an object of the invention to provide a multicarrier transmission system having a low power sleep mode and a rapid-on capability.
Further, it is an object of the invention to provide a multicarrier transmission system for use in digital subscriber line communications that can rapidly switch from a sleep mode to a full-on condition.
Still another object of the invention is to provide a DSL system that can readily be integrated into a computer having a low power sleep mode and which is capable of rapid return to full operation.